1. Field of the Invention
This invention relates to an improved process for producing acetic acid by carbonylation of methanol.
2. Technical Background
Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al. on Oct. 30, 1973. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter such as methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and the exact nature of the rhodium moiety within the active catalyst complex is uncertain. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
A major improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in U.S. Pat. No. 5,001,259 (issued Mar. 19, 1991); U.S. Pat. No. 5,026,908 (issued Jun. 25, 1991); and U.S. Pat. No. 5,144,068 (issued Sep. 1, 1992) and European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. These patents disclose a process in which acetic acid is produced from methanol in a reaction medium containing methyl acetate, methyl halide, especially methyl iodide, and a catalytically effective concentration of rhodium. The inventors of these patents discovered that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4 weight (wt) % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt % or 15 wt % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, a specified concentration of iodide ions over and above the iodide content that is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt %, so low that it can be defined simply as “a finite concentration” of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. its resistance to catalyst precipitation, especially during the product recovery steps of the process. Distillations carried out in the process to recover the acetic acid product tend to remove carbon monoxide ligands from the catalyst. These ligands have a stabilizing effect on the rhodium in the environment maintained in the reaction vessel. U.S. Pat. Nos. 5,001,259, 5,026,908 and 5,144,068 are incorporated herein by reference.
It has also been found that although a low water carbonylation process for the production of acetic acid reduces such by-products as carbon dioxide, hydrogen, and propionic acid, the amount of other impurities, present generally in trace amounts, is also increased, and the quality of acetic acid sometimes suffers when attempts are made to increase the production rate by improving catalysts, or modifying reaction conditions. These trace impurities affect the quality of the acetic acid product, especially when they are recirculated through the reaction process. See Catalysis of Organic Reactions, 75, 369–380 (1998), for further discussion on impurities in a carbonylation reaction system.
The crude acetic acid product is typically distilled in one or more distillation columns to remove light ends reaction components (typically methyl acetate and methyl iodide), water and heavy ends impurities. It has previously been observed that it is particularly important to avoid refluxing large amounts of methyl iodide back into the light ends distillation column because the separation of light ends reaction components from acetic acid product is significantly degraded if methyl iodide is allowed to reflux back into the light ends column. Ordinarily the refluxing of methyl iodide is prevented by separating most of the methyl iodide from the light ends overhead as a distinct phase, but under certain conditions the light ends overhead can form a single liquid phase that includes methyl iodide. The present invention provides one method of preventing this single-phase condition in the light ends column overhead.